There are a number of available optical gyroscopes. Resonant fiber optic gyroscopes (RFOGs) operate by recirculating a number of times a laser beam in each direction in a fiber optic ring. When the gyroscope is steady, the two counter-rotating beams are resonant at the same frequency. When the gyroscope moves the resonant conditions change so that each beam has a different resonant frequency. The difference between these shifted frequencies is a measure of gyroscope rotation rate. RFOGs require sophisticated electronics to measure this rotation rate which must be further processed to obtain the actual rotational angle. Drift caused by backscattering and other error sources requires further sophisticated electronic processing.
Interferometer fiber optic gyroscopes (IFOGs) also use two counter-rotating laser beams but they just circulate once in the fiber optic ring. The two beams create an interference pattern. The variation of the intensity of the fringes of the interference pattern represent the phase shift due to the movement of the gyroscope. Since the beams make only one circuit of the ring a long length of fiber optic element, a kilometer or more, is required and such elements are expensive. This system also requires sophisticated electronics to process the phase shift signal in order to obtain a measure of gyroscope rotation rate which must be further processed to obtain the rotation angle.
Ring laser gyroscopes (RLGs) also circulate two laser beams in opposite directions around a laser cavity. When the ring is stationary the beams oscillate at the same frequency. When the ring rotates the beam frequencies diverge and the difference in frequency is a function of the gyroscope rate of motion. Backscattering in RLGs causes locking problems which result in non-linear responses at low gyroscope rotation rates and must be compensated for by, for example, dithering the gyroscope and employing additional electronics to accommodate the dither. RLGs require costly, precision optics for the cavity and the mirrors to minimize backscattering and require high voltage to drive the HeNe laser source.